1. Field of the Invention
The present invention relates to a focus detecting apparatus for detecting a focus state of a taking lens of a camera or the like, with respect to an object to be photographed.
2. Description of Related Art
In a known focus detecting apparatus in a single lens reflex camera or the like, bundles of rays transmitted through different portions of an exit pupil of a taking lens are converged onto a pair of line sensors by an image reforming lens of a focus detecting optical system, so that the focus state of the taking lens can be detected in accordance with a relationship between outputs of the line sensors. The principle of the focus detection by the focus detecting apparatus is disclosed, for example, in U.S. Pat. No. 4,636,624.
A focus detecting apparatus having an off-axis detecting area, that deviated from an optical axis, is a known apparatus for detecting the focus state of a taking lens with respect to an object located at a position other than the center of an image plane.
However, if the deviation of the off-axis detecting area from the optical axis increases, vignetting focus detecting optical system pupil occurs when position or size of the exit pupil of the taking lens changes as a result of a lens change (i.e., when using an interchangeable lens), or during a zooming operation. In such cases, the area of the focus detecting optical system pupil, through which the bundle of rays is transmitted, is reduced. As a result, the applicability of such an automatic focus detecting system to an interchangeable lens is restricted, or it is possible that the automatic focus detecting system will not work at a specific focal length within a zooming range.
To solve these problems, it is known to provide three image reforming optical systems using three bundles of rays transmitted through three different portions of the exit pupil of the taking lens in a focus detecting apparatus having an off-axis detecting area, as disclosed for example in Japanese Kokai patent publication No. HEI 2-58012. In this focus detecting apparatus , the outputs of line sensors, corresponding to two of three image reforming optical systems in which no vignetting occurs, are used to detect an focus with respect to the object within the image plane. Accordingly, the adverse influence of vignetting on the focus detecting optical system, due to the change of the exit pupil of the taking lens, is eliminated.
However, in the known focus detecting apparatus, as mentioned above, the three image reforming optical systems must be provided in a single detecting area and the three line sensors must be arranged along a diametric direction of the taking lens.
Furthermore, if the area to be detected is large, namely, is an image is long, vignetting may also occur in an area including the optical axis of the taking lens. The prior art mentioned above does not solve this problem.